West African Coastal Area: Challenges and Outlook - The Land/Ocean Interactions in the Coastal Zone of West and Central Africa - page 19

variability of surface water temperatures in the Gulf of
Guinea and the dipolar structure of precipitations between
the Sahel region and the Guinean coast.

on the projected the frequency of extreme events (including
cyclones) as a result of global warming. There seems to be
more of an agreement on their increase in intensity because
of a perceptible rise in the temperature of marine surface
waters. Furthermore, storm surges depend greatly on local
conditions, in particular bathymetric and related to tidal
regimes. This means studies of storm surge statistics are
specific to each region and cannot be generalised.

Future Projections to 2050

Temperatures: The projections for Africa show temperature
rises that will very probably be well above the average
global rise, with an accentuation on the arid zones (AC-
MAD, in SDLAO, UEMOA-IUCN 2011 ).
Precipitations: The global ocean-atmosphere models
have more difficulty simulating precipitations than temper-
atures. In several regions of the world, these models agree
on the rise or fall in precipitations, but they diverge greatly
in their projection of precipitations in West Africa and the
signal for variations in precipitations on the Sahel and
Guinean coast remain uncertain. The overall average of the
various models presents a downward trend in precipitations
in JJA (June July August) on the West African coast to the
north of the 10th degree of latitude, which is approximately
the domain of the maritime trade wind and the north of the
Liberian-Guinean domain. This decline would be accom-
panied by an increase in the intensity of precipitations and a
decrease in the number of rainfall events (Tebaldi et al.
2006 ). In the south, on the domain of the permanent
Atlantic monsoon, the models do not agree on the signal of
change, even though the average presents a slight upward
trend.

Significant Wave Height

There are a limited number of studies on wave climatology
projections (Weisse and von Storch 2010 ). These studies
nonetheless allow for a considerable increase in the sig-
nificant height of waves in the North Atlantic, consistent
with the deviation of storm paths towards the poles. These
studies do not predict an upward trend in low latitudes. For
West Africa, the change will therefore come especially
from the increase in the frequency and duration of tidal
wave events, in particular related to extreme marine

weather events.

Higher Sea Level and Storm Surges

The fact that sea level is rising seems to have been largely
confirmed. The historical tide gauges show that in the
course of the past 100 years, the level of the sea has risen
by an average 20 cm. The current estimations are between
20 and 50 cm by the end of the century. Much more dra-
matic estimates evoke (on a conservative hypothesis) a rise
of 3.3 m according to a number of possible scenarios, such
as, for example, the complete disintegration of the West
Antarctic ice sheet (Bamber 2009 ).
The spatial distribution of the sea rise signal is none-
theless far from uniform. First of all, the surface of the
oceans is not regular and for example, in the subtropical
Atlantic, we note a convex area of approximately 1 m in
elevation. This spatial distribution also depends on climate
variability and the hazards of marine circulation. These
spatial disparities were already observed in the data for the
decade 1993-2003.
At regional level, this rise can significantly deviate from
the global average due to little known local factors such as
land subsidence, change in atmospheric circulation and
wind regime, the redistribution of atmospheric pressure or
the unequal distribution of thermal expansion. Our current
state

The increase in the intensity of precipitations and
the reduction in the return periods of certain extreme
events could cause the flooding of coastal zones and
aggravate erosion phenomena (which could in certain
specific cases lead to increased siltation). The global
reduction of rainfall in the course of the twenty-first
century would cause a decrease in the flow rates of the
major rivers such as the Senegal and the Volta, which
would be accompanied by a sediment deficit and an
aggravation of coastal erosion. To this should be
added the influence of works such as dams on these
watercourses, which only aggravate the trend/

phenomenon.

Frequencies of Extreme Events

of knowledge does not allow more accurate

Among the most important extreme events affecting the
coasts of West Africa, the episodes of intense precipitations,
depressions and tropical storms can cause considerable
damage. There is disagreement between the different studies

estimations.
According to the IPCC's 2007 report, in 2090-2099
average sea level will have risen by around 18-59 cm
compared to 1980-1999. By 2050, the rise will be in the